An analysis of a Folsom dam Tainter-gate which failed during operation in 1995 is presented. First, an empirical evaluation of the excessive push-and-draw hydrodynamic pressure induced by streamwise rotational vibration of the inclined circular-arc skinplate over the curved dam crest with a vertical step down at the upstream side is developed. This development is based on a theoretical analysis for a simplified flow field model. The simplified model consisted of a vertical flat rigid weir plate undergoing streamwise rotational vibration and a horizontal flat bed surface with a vertical step down at the gate position. Secondly, the resultant hydrodynamic load on the skinplate is determined. Thirdly, the dynamic instability of the Tainter-gate is theoretically examined and the major characteristics of this instability are presented. Finally, an FEM analysis of the structure reveals that the small vibration amplitude of at most 11.9mm at the spanwise center and bottom end of the skinplate induced the excessive hydrodynamic pressure load of about 1.6 times the static load, thus causing incipient failure of 4 bolts connecting a diagonal member to the main struts.

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